1. Field of the Invention
The present embodiments relate to batteries and more particularly to an apparatus and method for improving the performance of sodium-sulfur and sodium-iodide batteries.
2. Description of the Related Art
Our society has come to rely on batteries to power a myriad of devices, including computers, cell phones, portable music players, lighting devices, as well as many other electronic components. Nevertheless, there is an ongoing need for further advances in battery technology. For example, there is still a significant need for economical batteries that can power automobiles or provide load-leveling capabilities for wind, solar, or other energy technologies. (Such load-leveling capabilities may be used, for example, in power plants as a mechanism for storing the energy that is generated by the wind or solar generators.) Furthermore, the “information age” increasingly demands portable batteries that provide lighter weight, higher energy, longer discharge times, and smaller customized designs. To achieve these advances, technologists continue to work to develop batteries with higher and higher energy densities while still providing acceptable safety, power densities, cost, and other needed characteristics.
Sodium-sulfur (Na—S) batteries offer great potential to meet many of the above-stated needs. The theoretical specific energy of sodium-sulfur batteries is 792 Wh/kg, assuming the following overall reaction:2Na+3S→Na2S3 
This is one of the highest known specific energies for batteries that use non-gaseous constituents. The materials needed to produce these batteries are light, energetic, inexpensive, and readily available. In contrast with other types of positive electrode materials, sulfur is relatively non-toxic, thereby making these batteries relatively safe for human contact.
In light of this high specific energy, sodium-sulfur batteries have been commercialized into batteries that operate at elevated temperatures, e.g., at temperatures in excess of 250° C. and more typically between 300° C. and 350° C. Such sodium-sulfur batteries generally use one or more beta alumina membranes or beta” alumina membranes. These membranes require high temperature for good conductivity. Also the sodium negative electrode and sulfur positive electrode are molten at the elevated temperatures (e.g., between 300° C. and 350° C.) associated with these Na—S batteries.
Obviously, researchers have attempted to create sodium-sulfur batteries that can operate at lower temperatures (e.g., less than 250° C.). For example, U.S. Patent Application Publication No. 2010/0239893 discloses a sodium-sulfur battery which utilizes a ceramic alkali ion-conductive ceramic membrane and a solid alkali metal negative electrode (such as sodium). The battery taught by this patent application is designed to operate below 200° C. (The disclosure of U.S. Patent Application Publication No. 2010/0239893 is expressly incorporated herein by reference.)
However, there is still a need in the art for a new type of sodium-sulfur battery (and sodium-iodine battery) that can operate at lower temperatures but still can provide high power, high current density and other desired characteristics. Such a battery is disclosed herein.